Homozygosity for the CCR-5-delta-32 gene is linked to a high degree of protection from HIV-1 infection. The CCR-5-delta-32 allele carries a stable deletion of 32 bp which results in a truncated protein and in a frameshift that leads to a new sequence coding for a 31 AA carboxy terminus. The CCR-5-delta- 32 allele is present in a substantial percentage (~20%) of Caucasians, and could be the result of a genetic drift in the distant past that provided a selective advantage to undefined challenges. The genetic stability of CCR-5-delta-32, together with the absence of genetic polymorphisms at the boundaries of the 32 bp deletion, suggest that the gene product of the CCR- 5-delta-32 allele may exert a yet undefined protective function. The resistance to HIV-1 infection observed in CCR-5- delta-32 homozygous subjects has been thought to result from the absence of a functional CCR-5 protein that acts as a co-receptor for some HIV-1 strains. Alternatively, the gene product of the CCR-5-delta-32 allele could interfere with critical steps of the HIV-1 life cycle, resulting in a broad protection against different viral strains. To test this possibility, we constructed eukaryotic plasmids expressing the wild-type CCR-5 or the CCR-5-delta-32 allele, and used these to generate cell lines from CCR-5-delta-32 homozygous subjects that express the wild type CCR-5 allele together with CCR-5 wild-type cell lines expressing the CCR-5-delta-32 allele. These cells were challenged with different HIV-1 isolates. We found that cells from CCR-5-delta-32 homozygous individuals are at least one log less sensitive to infection by different HIV-1 strains. Consistent with this finding, the exogenous expression of CCR-5- delta-32 in wild-type cells resulted in a dramatic decrease in viral production, suggesting that the CCR-5-delta-32 protein can function as a negative transdominant regulator of viral expression. In transient expression experiments, the CCR-5- delta-32 protein down-regulated both the constitutive and the Tat-induced expression of a LTR-Luciferase reporter plasmid, indicating that the CCR-5-delta-32 protein may interfere with transcription of the HIV-1 genome. By flow cytometry and by confocal microscopy, the CCR-5-delta-32 protein was found in the cytoplasm, possibly associated with the cytoskeleton and with the inner cell membrane. These findings may shed some light on the mechanisms of resistance to HIV-1 infection of CCR-5-delta- 32 homozygous individuals, and may lead to the development of new therapeutic tools to prevent the establishment of HIV-1 infection and to control disease progression in HIV-1 infected subjects.